Olefin metathesis is considered as one of the most useful tools in organic chemistry. Since Grubbs reported the first generation ruthenium-catalyst (ref 1), numerous studies have been aimed at developing long life and more active catalysts and precatalysts. A breakthrough was the replacement of a phosphine ligand by a N-heterocyclic carbene (NHC), increasing the reactivity and stability of the corresponding complex (ref 2). See G-II in Scheme 1 below where PCy3 is tricyclohexylphosphine. Additional modifications afforded so-called boomerang-type catalysts, the most well-known being Hoveyda's catalyst (Hov-II in scheme 1 below) (refs 3,4). In such catalysts the benzylidene bears a donor atom that binds to Ru and decoordinates during catalysis to recoordinate after. More recently, replacing the benzylidene group of Grubbs original catalysts by an indenylidene moiety resulted in highly stable catalysts (for example M2 in scheme 1 where PCy3 is tricyclohexylphosphine) (ref 5).

This family of catalysts has proved its efficiency in various metathesis transformations, and studies on the variation of the NHC group have already been reported (ref 6).
Other examples of Ruthenium catalysts for use in metathesis transformations are described in U.S. Pat. No. 7,622,590.
Some cationic ruthenium complexes are known for use as catalysts and are shown in Scheme 1a below. For example Fürstner and Dixneuf (ref 7) have described 18-electron cationic allenylidene Ru complexes such as (a) below that were found to be catalyst precursors for ring closing metathesis (RCM). Hofmann (ref 8) describes dinuclear 16-electron cationic ruthenium complexes with chelating bisphosphane ligands (b), displaying activity in ring opening metathesis polymerization (ROMP). Kurosawa et al (ref 9) describe 18-electron cationic ruthenium complexes (c) made by chloride abstraction using silver salts. A latent cationic ruthenium NHC-based pre-catalyst (d) with excellent ROMP properties activated by UV irradiation has also been reported (ref 10). Complexes (e) are described by Romero et al (ref 11).

In view of the importance of olefin metathesis chemistry there remains the need to provide yet further metathesis catalysts. Some further catalysts are described in an earlier application (PCT/GB2011/000404) by the same inventor as the present application, the entire contents of which are incorporated by reference herein.